Optical glass or an optical transparent resin is used as a material for an optical device used in an optical system of various kinds of cameras, such as a camera, a one-time use camera and a video camera. Optical glass is excellent in heat resistance, transparency, dimensional stability, chemical resistance and the like, and includes various materials having various refractive indices and Abbe numbers, but has problems of high material cost, poor molding workability and low productivity. In particular, formation of an aspherical lens used for aberration correction requires extremely high technique and high cost, which result in a significant barrier on practical use.
In comparison with the above optical glass, an optical lens formed of an optical transparent resin, particularly a thermoplastic transparent resin, has such advantages that the lens can be mass-produced by injection molding and also can be formed easily into an aspherical lens, and thus the lens is currently applied to a lens for a camera. Examples of the resin include polycarbonate containing bisphenol A as a constitutional unit, polymethyl methacrylate and amorphous polyolefin.
In general, aberration of an optical system of a camera is corrected by the combination use of a plurality of convex lenses and concave lenses. Specifically, a chromatic aberration formed by a convex lens and a chromatic aberration of a concave lens that is opposite in sign to that of the convex lens are combined to negate the chromatic aberration. In this case, the concave lens is required to have a high dispersion (i.e., a low Abbe number).
In view of a high dispersion (low Abbe number) of the aforementioned optical thermoplastic resins, polycarbonate containing bisphenol A as a constitutional unit has a refractive index of approximately 1.59 and an Abbe number of approximately 32, polymethyl methacrylate has a refractive index of approximately 1.49 and an Abbe number of approximately 58, and amorphous polyolefin has a refractive index of approximately 1.54 and an Abbe number of approximately 56. Only polycarbonate may be used as a concave lens for aberration correction, but its Abbe number of 32 does not be considered to be sufficient to attain a high dispersion. Therefore, there is an increasing demand for a novel material that can be used as the concave lens for aberration correction.
Patent Document 1 discloses a polyester resin composition formed by copolymerization of a fluorene dihydroxy compound having a refractive index of 1.66 and an Abbe number of approximately 20 as a resin for use as a concave lens for aberration correction. The resin has a sufficiently large dispersion (i.e., a low Abbe number) but has the following defects as a resin for an optical lens. That is, the resin is formed by copolymerization of a large amount of a fluorene dihydroxy compound, which is bulky and rigid, and thus the resin has a considerably high melt viscosity and is poor in moldability. As a measure for improving the moldability, the melt viscosity upon molding may be lowered, i.e., the molding temperature may be increased, but the measure may cause such problems that coloration upon molding is intensified, and the mold is contaminated with thermal decomposition products. There is also considered such a measure that the melt viscosity is lowered by decreasing the polymerization degree, but the case involves such problems that the content of low molecular weight components in the resin is relatively increased to contaminate the mold with the low molecular weight components or decomposition products of the low molecular weight components. Thus, Patent Document 1 has failed to propose an optical lens formed of a thermoplastic resin that has both excellent optical characteristics (i.e., a high refractive index and a low Abbe number) and practically sufficient moldability.
Meanwhile, there is known a polyester produced from a naphthalenedicarboxylic acid and an aliphatic polycyclic diol. Patent Document 2 discloses a polyester produced from a naphthalenedicarboxylic acid, tricyclodecane dimethylol and ethylene glycol, which exhibits a high glass transition temperature, a high surface hardness, an excellent impact strength and an excellent transparency. In addition, Patent Document 3 also discloses a polyester produced from a naphthalenedicarboxylic acid, tricyclodecane dimethylol and ethylene glycol. However, in Patent Document 2, no study on optical applications of the polyester is made, and no optical properties of the polyester are described. On the other hand, in Patent Document 3, although there are described a refractive index and an Abbe number of the polyester, the polyester has an Abbe number as high as 26 or more and therefore fails to exhibit a sufficiently large dispersion (i.e., low Abbe number).
Patent Document 4 discloses the use of a polyester resin containing a unit derived from ethylene glycol, a unit derived from a diol having from 3 to 16 carbon atoms and a unit derived from a naphthalenedicarboxylic acid. However, it has been required that the polyester is further improved in moldability including a thermal stability or the like. Patent Document 4 also fails to describe a concrete example concerning the use of combination of the unit derived from a diol having from 3 to 16 carbon atoms such as tricyclodecane dimethanol and pentacyclopentadecane dimethanol with the other unit.